1. Field of the Invention
The present invention relates to a method for the at least temporary activation of bidirectional communication between a transponder and a base station. The invention relates further to a transponder that can be shifted at least to a first mute state and to a temporarily activated state.
2. Description of the Background Art
Bidirectional communication between a base station or a reader and a passive or semipassive transponder is used, for example, in contactless identification systems or so-called radio frequency identification (RFID) systems. This type of system typically consists of a base station or a write and/or read device or a reader unit and a plurality of transponders or remote sensors (tags), which are located simultaneously within the response range of the base station. The transmission of power and data between the base station and the transponder occurs either through inductive coupling or through coupling with the use of electromagnetic waves. The transponders or their transmitting and receiving devices usually do not have an active transmitter for data transmission to the base station. Transponders without active transmitters are called passive transponders when they do not have their own power supply and semipassive transponders when they have their own power supply. Passive transponders draw the power necessary for their supply from the electromagnetic field emitted by the base station or the emitted power.
Backscatter coupling is employed, as a rule, for data transmission from a transponder to a base station with ultrahigh frequency (UHF) or microwaves in the far field of the base station. To that end, the base station emits electromagnetic carrier waves, which are modulated and reflected by the transmitting and receiving device of the transponder by means of a modulation method in accordance with the data to be transmitted to the base station. Typical modulation methods are amplitude modulation, phase modulation, and amplitude shift keying (ASK) subcarrier modulation, in which the frequency or the phase position of the subcarrier is modified.
An access control method for transponders is described in the draft standard ISO/IEC_CD 18000-6C of 7 Jan. 2005. The transponder in this case is first selected from a number of transponders in a selection or arbitration process. The described selection process is a stochastic process in the form of a slot-based ALOHA protocol. Selection methods of this kind are described in detail, for example, in the handbook by Klaus Finkenzeller, RFID-Handbuch [RFID Handbook], 3rd ed., HANSER, 2002.
When the transponder is chosen or selected or singled out, the reader unit transmits a query to the transponder in the form of a retransmission of a random number previously transmitted by the transponder within the scope of the arbitration process, whereupon the transponder transmits protocol control bits (PC) and an identification in the form of a so-called electronic product code (EPC) to the reader unit. The protocol control bits contain information on a physical layer of the transmission path. The identification or the electronic product code EPC represents inter alia a product tagged with the transponder. The assignment of EPC to the tagged product is standardized, so that the product can be inferred from the EPC information. Furthermore, the EPC can be used by the reader unit as a pointer to other information, for example, to a password assigned to the EPC. The password can then be used to block memory areas in the transponder for write accesses.
The transmission of the EPC upon a query to the reader unit entails certain risks, however. Thus, during transport of products unauthorized persons can search selectively for products of a certain type, for example, for high-quality watches or weapons, because the product-identifying EPC is transmitted unprotected. To accomplish this, it is only necessary to bring a suitable reader into the transmission range of the transponders, for example, to a highway parking area or into train stations.
After the transmission of the PC and EPC by the transponder, read and/or write access to memory areas within the transponder by the reader unit is possible, unless the specific areas are blocked or locked for write access. Furthermore, read access to passwords can also be barred. Despite a possible read locking of passwords, there is some security risk that unauthorized persons have access to perhaps private memory contents.
When a transponder is used in so-called chip cards, personal data may be stored as memory contents. It is also desirable here to regulate access to these data, for example, so that upon entry into a store it cannot be determined automatically by reading the memory contents whether the particular customer still has funds on the chip card.
In the daily routine in industry as well, it may be desirable to mute a transponder at least temporarily. If, for example, transponders are attached to all products of a shipping unit, such as a pallet, it may be advantageous in logistics for identification and for management of the shipping unit when the transponders are muted at least partially. As a result, for example, recording becomes more rapid and efficient and multiple recording of certain information can be avoided. This in turn leads to a reduction of the computational effort in a so-called middleware for filtering functions or the like.
Muting of a transponder occurs, for example, by means of a so-called cloak or camouflage command. U.S. Pat. No. 6,690,264 B2 discloses a transponder, whereby the transponder comprises a circuit by which a switch can be actuated, so that communication via an antenna, i.e., transmission of data by the transponder, is enabled or prevented. Reception of data, on the contrary, is also possible in the mute state. This makes it possible to transmit a command to the transponder by which it is activated. An enable signal in this case is transmitted in plain text. This contradicts the general rule that passwords or the like at least in the forward channel may not be transmitted in plain text (i.e., from a base station to a transponder, or forward link), because the forward channel can be intercepted at a distance of 1 to 2 kilometers.